Thermal starting device for a single-phase asynchronous motor

ABSTRACT

The invention relates to single phase asynchronous motors in which a bimetal switch for opening the auxiliary circuit is in the circuit of the main winding of the motor. The bimetal switch is initially caused to open the auxiliary circuit by heat produced in a heating resistor in the auxiliary circuit. The bimetal switch is caused to remain open during operation of the motor by heat produced in the bimetal switch by reason of the current for the main circuit being directed through the bimetal switch.

United States Patent 91 Thorsoe [451 Nov. 27, 1973 {75] Inventor:

[ THERMAL STARTING DEVICE FOR A SINGLE-PHASE ASYNCHRONOUS MOTOR Fleming Thorsoe, Augustenborg, Denmark [73] Assignee: Danfoss A/S, Nordborg, Denmark 22 Filed: Aug. 14, 1970 21 Appl. No.2 63,941

Related U.S. Application Data [63] Continuation of Ser. N0. 684,664, Nov. 21, 1967,

abandoned. I

[52] U.S. Cl.. 318/221 11,318/221 C, 318/229 [51] Int. Cl. H021) 1/44 [58] Field of Search 318/220, 221, 221.3,

[56] I References Cited UNITED STATES PATENTS 2,132,888 10/1938 Werner 318/221 FOREIGN PATENTS OR APPLICATIONS 375.180 6/1932 Great Britain 318/221 Primary Examiner-Gene Z. Rubinson Attorney-Wayne B. Easton [5 7 ABSTRACT The invention relates to single phase asynchronous motors in which a bimetal switch for opening the auxiliary circuit is in the circuit of the main winding of the motor. The bimetal switch is initially caused to open the auxiliary circuit by heat produced'in a heating resister in the auxiliary circuit. The bimetal switch is caused to remain open during operation of the motor by heat produced in the bimetal switch by reason of the current for the main circuit being directed through the bimetal switch.

2' Claims, 3 Drawing Figures bimetal switch on completion of the starting operation.

In such starting devices, the time required by the bimetal switch to open during heating is utilized as the starting time. l-lere,'one has to distinguish between two types of heating. Firstly, the bimetal has-tobe heated during the starting period so that the switch opens. Se-

condly, during'the entire running period of the motor, a lower degree of heatinghas to be effected to keep the switch open. Whereas, wound wire heating resistors havebeen mainlyused in the past as heatingdevices, more recently thetendency is to avoid the use of such sensitivewire resistorsas much as possible. This, however, makes forconsiderable difficulties particularly as regards heating during the time that the motor is running. Direct heating of the bimetal by means of the current passing through was abandoned because the bimetalswitch was fitted in the circuit of the auxiliary winding and remained open during the running period, as a solid body, but special constructions are necessary for this if the heat output is to be kept small.

The object of the present invention, on the other hand, is to providea thermal starting device wherein no additional resistor at all is requiredfor heating the bimetal during the time that the motor is running.

In accordance; with the invention, this object is achieved by providing a bimetal, clamped at both ends, in the conductor. leading to the main winding and causing itto cooperate through a moving contact disposed in its middle part with a fixed contact contained in the auxiliary winding branch.

In this arrangement, the branching betweenthe. main winding circuit and the auxiliary winding circuit does not occur, as is usual, at a fixed branch point, but on the moving part of the bimetal. Consequently, main winding current continuously flows through the bimetal which is heated thereby during the entire periodthat' the motor is operating. Connection causes no difficulties, since the ends of the bimetal are firmly clamped. A further advantage resides in the fact that this method of heating is already fully effective at the. moment when the switch opens; any tendency of the bimetalswitch to close when the actual heatingfor startingup ceases, is thereby prevented.

It is true that the heating of the bimetal strip .by the current passing through takes effect during the starting period. There is no difficulty, however, in so selecting the resistance of the bimetal as to obtain, on the one hand, a definite starting time, and, on the other, optimum heating during the running period. For an asynchronous motor with an auxiliary resistance phase and a series resistor connected in series with the auxiliary winding, it is therefore recommended that theseries resistor should heat the bimetal. In particular, the series resistor can take the form of a solid resistor and can carry thefixed contact of the bimetal switch. In this arrangement'also, no wire heating resistor is required.

Theheat could, of course,.be at best connected through the fixed contact; the main part of the heat must be transferred by thermal radiation. The present construction is, however, very suitable for this, since the bimetal can extend parallel with a face of the solid resistor. Additionally, a quite considerable thermal output is produced in the series resistor, since it is not unusual to provide a series resistor connected in series with the auxiliary winding with 50 percent of the resistance value of the entire auxiliary phase. Such a series resistor should, however, receive at least 10 percent of this phase resistance value. Because of the double function of the series resistor, as a starting resistor on the one hand and as a heating resistor on the other, the capacity to be suppressed therein, is positively utilised. The solid resistor has. the further advantage that it stores heat and, after contact has been broken, continues togive off heat which still furtherreducesthe tendencyof the bimetal to return to the closed position. The arrangement described is particularly suitable for small encased refrigerating machines, in which the starting device is fitted outside the case. Here, part of the thermal output occurring during the starting operation is produced beforehand outside the case and therefore has no effect upon temperature conditions inside the case.

If the heat radiating on to the bimetal from the one side of the solid block should be insufficient, the bimetal can be clamped in metal carriers which extend along second side facesof the solid block. These metal carriers receive a" further part of the heat radiated from the solid resister. It is passed to the bimetal by conductance. Additionally, themetal carriers are very suitable for connection with the supply conductor. They also form a rigidsupportfor the bimetal.-

In a further form of theinvention, the bimetalswitch can be a snap-action switch. This has the advantage that rapidbreaking-tof the contact is ensured. The snapaction function can be inherent in the bimetal itself, if the metal carriers'act asarigid support.

To achieve uniform heating of the solid block, it is recommended that the solid resistor be metallisedon itsconnection' faces.

The inventionwill -now be described in'more detail'by reference to an embodiment illustratedin the drawing, wherein:

FIG. 1 is a circuit diagram for the starting device in accordance with the invention,

FIG. 2 shows a schematic section through the starting device in one form, and

FIG. 3-is a plan view of the device shownin FIG. 2.

An'electric motor (not shown in further detail) has an auxiliary winding 1' and a main winding 2, which are connected to mains terminals 4 through a common main switch 3. A thermal'starting device 5 consists of a'bimetalf6 with a movable contact 7 and a fixed contact 8, which is fitted on a solid resistor 9. The solid resistor-is contained in theauxiliary winding branch and serves as a heating resistor for the bimetal 6 as .well as a starting resistor, connectedin series, for the auxiliary winding 1. t

As can be seen from FIGS. 2 and 3, the bimetal 6 is clamped at'bothends in metal carriers 10 which may be made of brass, for example. The solid resistor 9 is madeof a ceramic resistor'material, ferrite, or the like andhas a metal coating 11 on two faces. Conductors connected toth'e metal carriers 10- and the solid resistor 9 are each marked with'an arrow, the referencenumerals of which indicate the associated element in the circuit.

When the main switch 3 is switched on, an almost constant current flows in the auxiliary winding branch, this current producing a considerable amount of heat in the solid resistor 9. The temperature of the solid block rises and radiates heat to the metal carriers and to the bimetal 9. When the bimetal reaches a certain temperature, it snaps into the open position, whereupon the starting operation and the heating of the solid resistor 9 are terminated. Irrespective of this, the main winding currents flows through the bimetal 6 during the whole period. This current suffices to supply the bimetal with an amount of heat such that the switch remains open for as long as the main switch 3 is closed and the motor is running.

The starting time can be adjusted in various ways, for example by varying the resistance value of the solid resistor 9 by means of increasing the distance between the carriers, 10 and the solid resistor 9, etc.

I claim:

l. A single phase asynchronous motor comprising a main circuit, an auxiliary circuit in parallel relation to said main circuit and containing an auxiliary starting winding, a bimetal element disposed in said main circuit having each end thereof fixedly clamped and having conductors of said main circuit extending from opposite' sides thereof, said element having a starting position and a normal running position, a fixed contact in said auxiliary circuit, a contact connected to said bimetal element engageable with said fixed contact when said element is in said starting position to'supply a starting current to said auxiliary circuit, a heating resistor in said auxiliary circuit in series with said fixed contact and said starting winding for heating said bimetal element, said element being movable from said starting position to said running position only by the combined effects of the heat produced by both said element and said heating resistor, said element being maintainable in said running position by the sole effect of heat produced by said element, said resistor being a solid resistor, said fixed contact being mounted on said resistor.

2. A motor according to claim 1 wherein said solid resistor has a metal plate on the side thereof on which said fixed contact is mounted and a metal plate on the opposite side thereof. 

1. A single phase asynchronous motor comprising a main circuit, an auxiliary circuit in parallel relation to said main circuit and containing an auxiliary starting winding, a bimetal element disposed in said main circuit having each end thereof fixedly clamped and having conductors of said main circuit extending from opposite sides thereof, said element having a starting position and a normal running position, a fixed contact in said auxiliary circuit, a contact connected to said bimetal element engageable with said fixed contact when said element is in said starting position to supply a starting current to said auxiliary circuit, a heating resistor in said auxiliary circuit in series with said fixed contact and said starting winding for heating said bimetal element, said element being movable from said starting position to said running position only by the combined effects of the heat produced by both said element and said heating resistor, said element being maintainable in said running position by the sole effect of heat produced by said element, said resistor being a solid resistor, said fixed contact being mounted on said resistor.
 2. A motor according to claim 1 wherein said solid resistor has a metal plate on the side thereof on which said fixed contact is mounted and a metal plate on the opposite side thereof. 